There is a vent line on many of these basket-shaped filters—the earliest models may not have them. The vent was designed to evacuate air bubbles that form in fuel that is agitated when at a low level in the tank. Air bubbles entering the pump can damage it. The best prevention is don't try to run the Brick on a spits-worth of fuel and fumes.
My Brick's pre-filter had one of those lines when I first looked into the fuel tank 10,000 miles after I bought. It was attached to the basket port and the upper end was detached. Once I understood its purpose, I removed it. During the 70,000 miles of riding afterward, no problems have occurred; however, it's a thoughtful feature on BMW's part for riders that cannot pay attention to details.
